1. Field of the Invention
This invention relates generally to magnetic transducers for reading information signals from a magnetic medium and, in particular, to a process for fabrication of improved conductivity leads for magnetoresistive read transducers.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks are commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which the MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni--Fe or Co or Ni--Fe/Co) separated by a layer of nonmagnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe--Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the information recorded on the magnetic medium (the signal field). In the SV sensors, SV resistance varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage.
In addition to the magnetoresistive material, the MR sensor has conductive lead structures for connecting the MR sensor to a sensing means and a sense current source. Typically, a constant current is sent through the MR sensor through these leads and the voltage variations caused by the changing resistance are measured via these leads.
FIG. 1 shows a prior art AMR sensor 100 comprising end regions 104 and 106 separated by a central region 102. Soft film 112, spacer layer 114, MR layer 110 and cap layer 116 are formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the MR layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current I.sub.S from a current source 160 to the MR sensor 100. Sensing means 170 connected to leads 140 and 145 sense the change in the resistance due to changes induced in the MR layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk).
FIG. 2 shows a prior art SV sensor 200 comprising end regions 204 and 206 separated by a central region 202. A free layer (free ferromagnetic layer) 210 is separated from a pinned layer (pinned ferromagnetic layer) 220 by a non-magnetic, electrically-conducting spacer 215. The magnetization of the pinned layer 220 is fixed by an antiferromagnetic (AFM) layer 221. Cap layer 208, free layer 210, spacer layer 215, pinned layer 220 and AFM layer 221 are all formed in the central region 202. Hard bias layers 230 and 235 formed in the end regions 204 and 206, respectively, provide longitudinal bias for the MR free layer 210. Leads 240 and 245 formed over hard bias layers 230 and 235, respectively, provide electrical connections for the flow of the sensing current I.sub.S from a current source 260 to the MR sensor 200. Sensing means 270 connected to leads 240 and 245 sense the change in the resistance due to changes induced in the free layer 210 by the external magnetic field (e.g., field generated by a data bit stored on a disk).
The preferred material for constructing the leads in both the AMR sensors and the SV sensors is a highly conductive material such as a metal. In the MR sensors conductor leads face much more stringent requirements when compared to other interconnect conductors, such as for example semiconductor devices. This is because the conductor leads, as well as the MR sensor films, are exposed at the sensor's air bearing surface (ABS). The leads have little protection from the severe mechanical environment where head-disk contact occurs frequently, and from the severe corrosion environment where chemical attack occurs both during processing and also in actual use where the environment may not be well controlled.
Early MR sensors were fabricated using pure gold metallurgy and other highly conductive materials as the lead conductor. However, due to the exposure at the ABS, these soft materials had the potential reliability risks of electromigration, smearing and nodule formation. Tungsten and tantalum were introduced as gold substitutes due to their mechanical properties of being very hard while still having good electrical conductivity.
Referring again to FIG. 2, the electrical resistance of the leads 240, 245 connecting the SV sensor 200 to the sense current source 260 is an important factor influencing operating conditions of the SV sensor and its sensitivity as a read 5 transducer. High lead resistance can result in excessive IR heating of the SV sensor structure when lead resistance becomes significant. A second problem of high lead resistance is that the effective SV sensor sensitivity to the signal field is degraded when lead resistance becomes a significant fraction of 10 the total resistance measured by sensing means 270.
The resistance R.sub.L1 of lead 240, resistance R.sub.L2 of lead 245 and the resistance R.sub.SV of the central region 202 of the SV sensor 200 together form a series electrical circuit through which the sense current I.sub.S flows. The resistance through which sense current I.sub.S flows can be expressed as R.sub.T =R.sub.SV +R.sub.L where the total lead resistance R.sub.L =R.sub.L +R.sub.L2. It is desirable to have the sense current I.sub.S flowing through the SV sensor resistance R.sub.SV as high as possible to maximize the voltage signals measured by sensing means 270. The sense current I.sub.S flowing through R.sub.SV generates heat I.sub.S R.sub.SV which raises the temperature of SV sensor 200. Current I.sub.S flowing through the lead resistance R.sub.L generates additional heat I.sub.S R.sub.L which also contributes to the rise in temperature of the SV sensor 200. The allowable operating temperature for reliable SV sensor operation is about 120 degrees C. If lead resistance R.sub.L becomes a significant fraction of the total resistance R.sub.T, the lead resistance can become a limiting factor on the maximum allowable sense current I.sub.S.
The total resistance sensed by the sensing means 270 is R.sub.T =R.sub.SV +R.sub.L. The effect of signal field changes on the SV sensor 200 is a change of resistance by an amount of deltaR.sub.SV. The effective sensitivity of the SV sensor 200 as sensed by the sensing means 270 can be expressed by the ratio deltaR.sub.T /R.sub.T. Since lead resistance R.sub.L does not change with signal field, deltaR.sub.T is equal to deltaR.sub.SV. Thus the effective sensitivity of the SV sensor 200 as sensed by the sensing means 270 is given by deltaR.sub.SV / (R.sub.SV +R.sub.L). Comparison of this expression with the expression for the GMR coefficient deltaR.sub.SV /R.sub.SV of the SV sensor 200 shows that the effective sensitivity is significantly degraded as lead resistance becomes comparable to the SV sensor resistance.
In present SV sensors, the SV sensor resistance R.sub.SV is typically about 30 ohms. Lead resistance R.sub.L is typically about 15 to 18 ohms. The effective sensitivity is therefore degraded by about 100R.sub.L /(R.sub.SV +R.sub.L)=35%. For AMR sensors, typical sensor resistance is 30 to 50 ohms and lead resistances are 15 ohms, so that effective sensitivity is degraded by about 27%.
As higher data density disk drives are developed, smaller MR sensors with thinner lead layer structures are required to provide the higher data resolution. Lead resistance becomes an even greater fraction of the total MR sensor resistance in these smaller devices. Therefore, there is a need for low resistance leads that do not limit performance of present and future MR sensors.